It is known to include fatty or oleophilic materials, including silicones, in cosmetic products to provide occlusive (moisture-retention) properties, improved feel properties, as solvents and for other reasons. It is also known to include metal oxide particle benefit agents in cosmetic compositions—depending on the type of oxide, the particle size and/or configuration, agents may, for example, provide pigmentary and/or sunscreening benefits. In order to marry the benefits of both types of component, it is a natural step to consider including both types of component in a single composition. This presents difficulties, however, because metal oxide particles generally do not readily disperse in a hydrophobic matrix. To overcome this difficulty, it is known to stabilise metal oxide particles within an oleophilic phase by adding emulsifying agents to a composition.
Emulsifiers in low dielectric constant media may use steric effects to provide stabilisation and prevent flocculation. To be more precise, the emulsifier coats the free surface of the particle with hydrophilic tails and extends oleophilic chain into the medium and the chain acts to prevent agglomeration by osmotic effects: as two particles approach one another, the chains overlap and cause a temporary increase in polymer concentration. This increase causes an osmotic stress that forces fluid between the particles, thereby causing them to separate.
The addition of free emulsifier to a composition may, however, not be sufficient to ensure a good particle dispersion throughout the life cycle of the product. If the concentration of emulsifier is too low then the osmotic stress will be correspondingly low and, as two particles approach one another, the emulsifier may bridge the particles actually promoting agglomeration. If, on the other hand, the concentration of emulsifier is too high, then the osmotic stress behaviour may be reversed leading to depletion flocculation. In this scenario, the concentration of free emulsifier may be so high that, as particles approach one another, free emulsifier may be forced out from between them. The concentration difference may create an osmotic stress that draws fluid out of the space between the particles thereby promoting agglomeration.
An ideal level of emulsifier exists for any system but small changes in that system may cause the the amount of emulsifier to move away from the optimum, thereby leading to the above-described problems. Especially in the case of a product that is required to dry down in use, it is almost impossible to achieve an ideal level of emulsifier at all time points, because, during the drying process, the emulsifier concentration continually increases. In other words, the use of non-bonded coating on the surface of the particle means that for any system the emulsifier concentration must be sub-optimal at some stage in its life cycle.
To overcome the disadvantages of non-bonded emulsifier coatings, use of a coating which is bonded to the surface of a particle may be employed. Bonding may prevent the emulsifier bridging and, since the there is no free emulsifier in the solution, depletion flocculation may be avoided. In addition, since it is no longer necessary to prevent diffusion of emulsifier away from the surface, there is no requirement to control the hydrophilic/lipophilic balance (HLB) of the emulsifier. As a result, the molecules in a bonded coating may have longer tails than non-bonded emulsifiers, which, in turn, may increase the steric stabilisation effect.
It is known to formulate compositions comprising metal oxide particles which have been coated with bonded emulsifier to provide steric stabilisation. Such formulations are disclosed in the article entitled “Development of Novel Silicones for Powder Surface Treatment” by Masaneo Kamei in the Fragrance Journal, p. 81-85, 2002-6. The cosmetic compositions disclosed in that article are difficult to apply to skin, however, which may result in the benefit achieved by the metal oxide particle benefit agent being less even than it should be.